Background: I have no background in medical sciences, and this is something I've been thinking about since the start of the Covid-19 pandemic; please ELI5 your answer if you can. Thanks!
We estimate the age of the Earth to be in the hundreds of millions of years old, and the age of human society to be in the tens of thousands of years. One would therefore posit, modulo changes in environment, that pretty much every naturally occurring virus that exists, has probably been discovered, or at the very least, has been experienced and has some kind of history; even if we didn't know what it was at the time, we could at least look back at historical records and go "oh, that was that disease, now we know what it is and can treat it".
When the Covid-19 pandemic began, scientists were not sure if it was natural or made in a lab, and to the best of my knowledge that question has still not been conclusively answered a year later (I may be unaware of that being decided). My question is: Why do we believe that the Covid-19 virus is (possibly) naturally occurring, given that for the tens of thousands of years of human history, prior to 2019, we have no evidence (to my knowledge) of it ever being encountered, and it is sufficiently different from anything we had seen before that we (humans as a whole) were unequipped to deal with it sufficiently (reducing the probability of it being a recent mutation of something we had seen before)? What sorts of precedents exist for this sort of situation, where a very virulent, potent, and communicable disease has existed in nature for a long period of time, but has never been discovered until "recently" (for some definition of "recently" in context)?